Undesirable power line transients have the potential to damage sensitive electrical equipment present on the line. Transients can be caused by external sources of energy such as a lightning strike, energy from an inductive load fed back to the line, or from power line switching. In order to protect the loads on the power line, this additional energy must be either dissipated or stored and later returned to the line.
Known voltage regulators and transient protection devices typically rely on shunting some or most of the extra energy into the ground return of the device being protected. Due to magnitude of the current that must be diverted into the ground circuit during a transient suppression, the "ground potential" of the protected device is raised well above true ground. In some cases the ground potential is raised due to the current flowing through the inductance in the ground return from the device being protected. This problem is especially apparent with equipment located in two different buildings, where the whole ground potential may raise due to true earth ground impedance.
Raising the potential of the protected device is not in and of itself a problem. However, if anything else is connected (e.g. by a communication cable) to the protected device that also has a reference to earth ground, excessive current may flow between the two devices through the communication cable. For example, in a computer connected to other devices such as a modem, printer, monitor, test equipment, another computer, or any other circuit that also has a reference to earth ground, the raised ground potential may cause an excessive current to flow between the communication cables connecting the equipment.
A known transient protection system is described in U.S. Pat. No. 5,448,443 to Muelleman. Muelleman adds inductance in series with the ground return. This added inductance further raises the ground potential during a transient, thereby amplifying the problem of unwanted and dangerous levels of current traveling down data lines and thus through the electronics which need to be protected.
Communications cables, such as monitor and keyboard cables, typically include a small filter designed to limit high frequency electromagnetic or radio frequency interference (EMI/RFI). These small filters do not have sufficient capacity to limit transient signals and saturate after only a small volt-time input.